Photosynthesis is important to animals because it produces the two things they need most: oxygen to breathe and glucose to fuel every cell in their bodies. Without this single chemical process, animal life as we know it could not exist. Plants, algae, and certain bacteria capture sunlight and convert carbon dioxide and water into sugar and oxygen, creating the foundation that the entire animal kingdom depends on.
It Produces the Oxygen Animals Breathe
Every animal on land and in the sea relies on oxygen to convert food into usable energy. That oxygen comes almost entirely from photosynthesis. Roughly half of Earth’s oxygen is produced by oceanic plankton, drifting algae, and photosynthetic bacteria. One species of ocean bacteria alone generates up to 20 percent of the oxygen in the entire biosphere, more than all the tropical rainforests on land combined, according to NOAA. The other half comes from terrestrial plants and trees.
Once an animal inhales that oxygen, it enters the mitochondria of cells, where it’s used in a process called cellular respiration. This is essentially photosynthesis running in reverse: oxygen and glucose are broken down into carbon dioxide, water, and ATP, the molecule cells use as energy currency. Without a constant supply of photosynthetically produced oxygen, this process stops, and cells can no longer power themselves.
It Creates the Food That Powers All Animal Life
Photosynthesis doesn’t just release oxygen as a byproduct. Its primary purpose is to convert solar energy into chemical energy, storing it in the carbon bonds of glucose. That glucose is the original energy source behind virtually every meal an animal eats. When you eat a piece of fruit, you’re consuming glucose that a plant built directly from sunlight. When you eat a steak, you’re consuming energy that a cow obtained from grass, which built its glucose through photosynthesis.
This is why plants and algae are called primary producers. They sit at the base of every food chain on the planet. Herbivores eat the producers directly. Carnivores eat the herbivores. Apex predators eat the carnivores. At every step, the energy being passed along originated in photosynthesis. Remove that first link and the entire chain collapses. All animal species on Earth are consumers, and they depend on producer organisms for their food.
The numbers are striking. Humans alone command roughly 40 percent of the total plant productivity on land. That figure accounts for the crops we grow, the forests we harvest, and the land we convert for agriculture. Every calorie we eat traces back to a photosynthetic organism somewhere in the chain.
How Carnivores Still Depend on Plants
A wolf never eats a leaf, yet it is completely dependent on photosynthesis. The connection works through trophic levels, the steps in a food chain. A wolf eats a deer. The deer ate grasses and shrubs. Those plants captured sunlight and built glucose. The wolf is two steps removed from photosynthesis, but without it, there are no grasses, no deer, and no wolf.
This indirect dependence applies to every carnivore and omnivore. A shark eats fish that ate smaller fish that ate zooplankton that ate phytoplankton. A spider eats flies that fed on decaying plant matter. No matter how many links separate a predator from a plant, the energy flowing through its body was first captured by photosynthesis. Without organisms that can fix light energy into food, there would be no energy available to any organism that lacks that capability.
It Regulates the Atmosphere Animals Live In
Beyond producing oxygen, photosynthesis pulls carbon dioxide out of the atmosphere. This matters for animals in a less obvious but equally important way: it helps regulate the climate they live in. Carbon dioxide is a greenhouse gas, and its concentration directly affects global temperatures, weather patterns, and ocean chemistry.
Through photosynthesis, ecosystems naturally sequester and store carbon. Forests, grasslands, and ocean phytoplankton absorb billions of tons of CO2 each year, keeping atmospheric concentrations lower than they would otherwise be. When this carbon removal slows, through deforestation or ocean warming, temperatures rise and habitats shift. Coral reefs bleach. Migration patterns change. Species lose the stable conditions they evolved to thrive in.
Over the past 100 years, atmospheric oxygen has declined by about 0.1 percent due to fossil fuel burning, which consumes oxygen and produces carbon dioxide. That’s a small number in absolute terms, but it’s fast by geological standards. Over the previous 800,000 years, oxygen declined by only 0.7 percent total. Photosynthesis is the primary mechanism working to replenish what’s lost.
Some Animals Host Photosynthesis Directly
Most animals benefit from photosynthesis indirectly, by breathing the oxygen and eating the food it produces. But some animals have taken the relationship a step further by hosting photosynthetic organisms inside their own tissues.
Reef-building corals are the most well-known example. Corals harbor tiny algae called zooxanthellae within their cells. These algae photosynthesize and provide the coral with most, if not all, of the energy needed to meet its metabolic demands. In return, the coral provides the algae with shelter and access to nutrients. This partnership is what allows coral reef ecosystems to thrive in nutrient-poor tropical waters that would otherwise be too barren to support such dense life. Beyond corals, the same type of algae lives in symbiosis with sea anemones, flatworms, mollusks, and sponges.
These symbiotic algae also help with nutrient recycling. Some act as a sink for animal metabolic waste, particularly ammonia, and then recycle that nitrogen into compounds the host animal can use, such as essential amino acids. The animal gets both energy and nutrients from the algae living inside it.
The Deep-Sea Exception
There is one environment where animal life exists without any connection to photosynthesis. Deep on the ocean floor, around hydrothermal vents and cold seeps, entire ecosystems run on chemical energy instead of sunlight. Bacteria at these sites use a process called chemosynthesis, converting chemicals like hydrogen sulfide into energy. Tube worms, crabs, shrimp, and other animals then feed on these bacteria or host them internally.
These communities were first discovered in the late 1970s and challenged the assumption that all life ultimately depends on sunlight. Chemosynthetic bacterial communities have since been found around hot springs on land, whale carcasses on the seafloor, and even sunken ships. They remain rare exceptions, though. The vast majority of animal life on Earth, on land and in the oceans, traces its energy supply back to photosynthesis.